Modern digital multimeters (DMMs) can suffer from ghost voltages. Ghost voltages can occur when energized circuits and non-energized wiring are located in close proximity to each other, such as in the same conduit or raceway. This condition forms a capacitor and allows capacitive coupling between the energized wiring and the adjacent unused wiring. When multimeter leads are placed between the open circuit and the neutral conductor, the circuit is effectively completed through the input of the multimeter. The capacitance between the connected, hot conductor and the floating conductor forms a voltage divider in conjunction with the multimeter input impedance. The multimeter then measures and displays the resulting voltage value, which can be misleading.
Most modern DMMs for testing industrial, electrical, and electronic systems have high impedance input circuits greater than 1 megohm. As a result, when the DMM is placed across a circuit for a measurement, it will have little impact on circuit performance. This is the desired effect for most voltage measurement applications, and is especially important for sensitive electronics or control circuits. However, this high impedance can contribute to the ghost voltage effect. Many DMMs have an input impedance that's high enough to show the capacitively coupled voltage, giving a false impression of a live conductor. That is, the meter is actually measuring voltage coupled into the disconnected conductor.
To compensate for this effect, DMMs can include a low impedance function, often called “LoZ.” This feature presents a low impedance input to the circuit under test. This reduces the possibility of false readings due to ghost voltages and improves accuracy when testing to determine absence or presence of voltage. The LoZ switch position on the DMM can be used when readings are suspect (ghost voltages may be present) or when testing for the presence of voltage. Typical implementations of LoZ functionality pass current through a resistor. In LoZ mode, the resistor can generate a significant amount of heat, and this heat can adversely impact the accuracy of further measurements taken with the DMM.
The headings provided herein are for convenience only and do not necessarily affect the scope or meaning of the claimed invention.